1. Field of the Invention
This invention relates to a superconductor. More particularly, the present invention is concerned with an oxide-superconductor improved so as to have a high critical temperature (T.sub.c) and a process for preparing the same.
2. Description of the Related Art
Oxide-superconducting materials having a perovskite crystalline structure or a K.sub.2 NiF.sub.4 crystalline structure similar to the perovskite structure, i.e., Ba.sub.x La.sub.5-x -Cu.sub.5 O.sub.5(3-y) (wherein x=1 or 0.75 and y&gt;0), were discussed in Z. Physik. B-Condened Matter 64, 189-193 (1986). Besides the above-described materials, among oxide superconducting materials having a perovskite crystalline structure, SrTiO.sub.3 and BaPb.sub.1-x Bi.sub.x O.sub.3 were discussed in Physical Review, Vol. 163, No. 2, Nov. 10, 380-390 (1967) and Appl. Phys. 22, 205-212 (1980), respectively.
Therefore, it was expected that beside these materials there would exist oxides having the above-described crystalline structure exhibiting superconductivity which were considered to be useful materials capable of constituting a superconductive device. However, the critical temperature, T.sub.c, of the oxide superconducting material having a perovskite crystalline structure is as low as 35 K in the case of Ba.sub.x La.sub.5-x -Cu.sub.5 O.sub.5(3-y) (wherein x=0.75 and y&gt;0), and no materials exhibited superconductivity at a temperature exceeding this temperature.
Similarly, an oxide-superconductor having a high superconducting critical temperature, i.e. (Sr,La).sub.2 CuO.sub.4, is discussed in Phys. Rev. Lett., 26, 408-410 (1987).
Moreover, a Y-Ba-Cu oxide system is known as a superconductor having a critical temperature, T.sub.c, exceeding the liquid nitrogen temperature (77 K).
All the above-described superconductors have a structure obtained by modification of a perovskite structure which is relatively stable. Specifically, (Sr,La).sub.2 CuO.sub.4 has a K.sub.2 NiF.sub.4 structure while the Y-Ba-Cu oxide system has a more complicated structure, i.e. an oxygen-deficient perovskite structure. A.sub.3 B.sub.2 O.sub.7, A.sub.4 B.sub.3 O.sub.10, A.sub.5 B.sub.4 O.sub.13, and A.sub.6 B.sub.5 O.sub.16 structures are considered to be further more complicate. In general, the more complicate the crystalline structure of the material, the more difficult the production of the material. Therefore, the difficulty in producing the above-described materials increases in order of the materials described above. This raised a problem that the difficulty in obtaining a homogeneous material would increase in order of the materials described above and the superconducting critical current density would decrease in that order. In general, the superconducting critical temperature of the oxide-superconductor characterized by possessing a CuO.sub.6 octahedron like the above-described materials is higher when the crystalline structure is more complicate and the planes in which the c-axis is defined as the normal have higher two-dimensionality, i.e. an interaction between these planes are decreased. This is because an increase in the two-dimensionality brings about a shortening of the a-axis and a decrease in the distance between the Cu atom and the O atom within the plane. This causes an increase in the degree of overlapping between the 3d orbital of the Cu atom and the 2p orbital of the O atom, thus leading to an improvement in the conductivity of carriers within the plane. Therefore, the superconductors comprising the above-described materials have a high superconducting critical temperature. However, they have complicate crystalline structures, which leads to a problem that it is difficult to produce them.
In general, in the oxide-superconductors characterized by possessing a CuO.sub.6 octahedron or a CuO.sub.5 pentahedron like the above-described materials, the number of the carriers are determined by the composition ratio of the A site atoms, e.g. the composition ratio of Sr to La or that of Ba to Y in the case of the above-mentioned materials. Therefore, the electron density of state, N(0), of the Fermi-surface is also determined by the composition ratio. According to the Bardeen-Cooper-Schrieffer theory (abbreviated to "BCS theory"), the superconducting critical temperature, T.sub.c, is represented by the following equation: EQU k.sub.B T.sub.C =1.14h.omega..sub.D x exp(-1/N(0)/V)
wherein .omega..sub.D is a Debye temperature and V a parameter representing the magnitude of the electron-phonon interaction. Therefore, in order to increase the superconducting critical temperature, T.sub.c, it is necessary that the electron density of state, N(0), of the Fermi-surface be large. For this reason, it is necessary to maximize the electron density of state, N(0), of the Fermi-surface by changing the number of the carriers through proper change of the A site atoms. However, in the above-described materials, the parameter capable of changing the number of the carriers is only the composition ratio of the A site atoms, i.e. the composition ratio of Sr to La or that of Ba to Y, which leads to a problem that the number of the parameters is not sufficient to maximize the electron density of state, N(0), of the Fermi-surface.
A composite oxide having a composition comprising a sesquioxide of lanthanum containing an alkaline earth metal (M) and a copper (Cu) ion which is represented by the formula (La.sub.1-x M.sub.x).sub.2 CuO.sub.4 (wherein M is Sr, Ba or Ca) has a nickel potassium fluoride (K.sub.2 NiF.sub.4) crystalline structure. This is a crystal belonging to a tetragonal system (space group: D.sub.4n -I4/mmm) and has a composite structure comprising a rock salt structure and a perovskite structure which are put on top of each other to form a multi-layer structure. On the other hand, a composite oxide of a Y-M-Cu oxide system (wherein M is Sr, Ba or Ca) comprising a sesquioxide of a rare earth element containing an alkaline earth metal (M) and a copper (Cu) ion has a perovskite-like crystalline structure. The rare earth element is present as an ion spacer between both structures, and the copper ion is within an oxygen octahedron. Therefore, this structure causes a difference in the characteristics in respect of electrical conductivity and electron emission between the direction of the c-axis and the plane normal to the c-axis, i.e. causes anisotropy. In recent years, it has been proved that a composite oxide prepared by replacing part of lanthanum with a small amount of an alkaline earth metal, such as barium or strontium, exhibited superconductivity at 30 to 40 K or a higher temperature, i.e. was a superconductor having a high critical temperature. In this kind of superconductor, there exists a difference by a factor of 10 in the coherence length of the superconductivity between the direction of the c-axis and the direction normal to the c-axis. Further, this composite oxide can contain barium and strontium as the constituent element, M, which can form monoatomic layers capable of reducing the work function on the surface thereof through combination with an oxidized atom, which also renders this composite oxide suitable as an electron emitting material.
However, the above-described material exhibits anisotropy because the monoatomic layers capable of reducing the work function are laminated in the direction of the c-axis. Therefore, a single crystal of this composite oxide is required in order to put this composite material to practical use as an electrically conductive material. This composite oxide comprises, e.g., La.sub.2 O.sub.3, Y.sub.2 O.sub.3 and CuO, BaO or SrO which tend to form a mixed phase when heated to around the melting point. Therefore, no excellent single crystal can be obtained by the crystal pulling method which requires the dissolution at a high temperature. Under these circumstances, hitherto, there have been proposed no process for preparing a composite single crystal which can be put to practical use.